The invention relates to a temperature sensor with a temperature-sensitive element on a surface of a monocrystalline substrate, wherein the temperature-sensitive element is made of a platinum thin-film resistor. The invention further relates to a process for manufacturing such a temperature sensor as well as a use for it.
Temperature sensors with a platinum thin-film resistor on a monocrystalline substrate are known. Thus, Russian Patent RU 2069324 describes a temperature-measuring device with a dielectric substrate and a thin-film resistor applied thereon in the form of a meander made of nickel or platinum. Between the platinum layer and the substrate is a layer of adhesive agent made of titanium nitride. Sapphire, for example, has been disclosed as a dielectric substrate.
U.S. Pat. No. 6,229,121 B1 discloses a micro switch with a bimetallic switch contact that has a meander-shaped heating element on an epitactic layer and a temperature-sensitive element, for example as a platinum thin-film resistor.
International Patent Publication WO 87/05146 describes a temperature sensor with an insulated substrate, for example made of sapphire. A platinum thin-film resistor is provided as a temperature sensor which is protected with a cover layer.
German Published Patent Application DE 32 05 704 A1 discloses a device to evaluate the climate in a room using a temperature sensor element, which has a platinum thin-film resistor in the form of a meander on a substrate made of sapphire, for example.
U.S. Pat. No. 4,378,489 describes a measuring arrangement with a platinum thin-film resistor and a heating element on a sapphire support.
In order to be able to attain a further geometric size reduction of temperature sensors containing platinum thin-film resistors, ever thinner and longer conductor paths must be produced on smaller and smaller substrate surfaces. The crystalline platinum thin-film resistors produced up to this point for temperature sensors are technically feasible beginning at a conductor path width of approximately  greater than 10 xcexcm, since at a smaller width the conducting electrons leak at the platinum grain boundaries and the temperature coefficient cc of the electrical resistance of platinum vanishes. In addition, in structuring the crystalline platinum thin-film resistor, for example by etching, voids can occur in the conduction path, which can be traced back to the platinum crystallites, their shape, size and orientation.
Therefore, the problem becomes one of producing a temperature sensor with a platinum thin-film resistor as a temperature-sensitive element in which it is possible to reduce the conductor path width and conductor path separation of the platinum thin-film resistor to  less than  about 10 xcexcm. Moreover, a suitable process to manufacture such a temperature sensor should be provided.
The problem is solved for the temperature sensor in that the platinum thin-film resistor is constructed as an epitaxial layer. An epitaxial layer is understood as a monocrystalline layer produced on a monocrystalline substrate by epitaxy. If the platinum thin-film resistor is constructed as an epitaxial layer, it is then very easy to etch due to its single crystal character, and error-free conductor path widths and path separations of  less than  about 10 xcexcm can be produced. The temperature coefficient xcex1 of the electrical resistance of the platinum remains intact, since there is no longer any leakage of conducting electrons at the grain boundaries. With the ability to construct thinner and thus longer conductor paths on a substrate, electrical resistors of  greater than  about 10 kxcexa9 can be attained for the platinum thin-film resistor without any problem.
The problem is solved for the process in that the epitaxial layer(s) is/are deposited by PVD (physical vapor deposition) or CVD (chemical vapor deposition) or MBE (molecular beam epitaxy).